It has been known that ultrasonic perforations of cured thermo set plastics tends to cause fracturing of the material. As a result, a prior perforating system described in U.S. Pat. No. 4,160,055 issued July 3, 1979 suggests that the perforations be accomplished in an uncured plastic material. The object of perforating uncured plastic is to prevent fracture by the utilization of softer, more pliable materials, which allow penetrating pins to easily perforate the material.
One of the drawbacks of the aforementioned system is its low throughput. By reason of using soft materials in the perforating process, such a system cannot process the material in a high speed continuous manner.
A continuous and high speed apparatus and method of perforating a strip of material is shown in U.S. Pat. No. 4,472,461 issued Sept. 18, 1984. Such a system perforates the material by moving the strip past a fixed gas ejecting cylinder or tube. The tube contains discrete, spaced-apart, gas emitting apertures that puncture holes in the uncured, adhesively coated material, as the material is caused to move past the tube.
While this system can accomplish a perforation of the strip, its perforating throughput is limited to approximately 50 feet of material per minute, or less.
The subject invention has discovered that a strip of material, such as cured plasticized thin film, can be continuously perforated by an ultrasonic horn without fracturing the plastic, and with an increased throughput of approximately up to 220 feet per minute. The system and method of the invention accomplishes the above result with solid, or cured plastic material. The strip of material is momentarily engaged between a knurled, rotating roller, and an ultrasonic horn as the material is caused to move past at the high linear speed of approximately 200 feet per minute. The ultrasonic horn disposed adjacent the rotating knurled roller, on the opposite side of the web of strip material, forces a rapid and momentary perforating contact between the strip and perforating projections of the knurled surface. The material maintains its structural integrity while being perforated at the perforating projections of the knurled roller.
A roll of plastic film, with a pressure sensitive adhesive coating on one side and said pressure sensitive coating covered by a coated release paper, was mounted onto a rotatable unwind shaft. The film web (with adhesive and protective covering) was led through a web-tensioning device to provide constant web tension throughout the ensuing operation. The web was passed through a perforating device and then rewound.
The perforating device is composed of a patterned roll and a platen surface.
The patterned roll may have a knurled surface or a spiked surface, each providing a multitude of contact points which produce an overall pattern of perforation. These surfaces may be prepared by knurling or by engraving.
To increase the useful life of the roll surface, hardening is recommended. Best hardness of the spike points was achieved by use of the E.D.M. method of engraving. The spikes can be shaped as pyramids or round spires. They may be pointed or slightly truncated.
The platen surface is a flat, hardened, carbide facing on the edge of a specially shaped steel plate. This combined part is referred to as the "horn." The horn is mounted in a vertical position and is attached to an amplifier.
When ultrasonic energy is applied to the amplifier, the horn vibrates internally with very small amplitude at very high frequencies. The horn and amplifier are mounted in a vertically traversing mechanism that permits precise positioning relative to the surface of the patterned roll.
However, the preferred procedure was to have the film on the downside, i.e., making first contact with the spires of the patterned, surface. The different registration of the horn may obtain different forms of perforations. A good result is obtained with the platen on the horn touching the paper. In such instance, the paper remained unbroken while the proper opening was formed in the plastic from the knurled roller whose spikes may be slightly truncated.
The strip is perforated on this mechanism and satisfactory, permanent "breathing" holes are formed in the laminate regardless of which side of the web is in contact with the perforating machine.
A hard roll (Rockwell C55-62) is preferred. The platen surface was positioned so that no contact is made with the spires when at full vibrating amplitude. In this position, the spires fully penetrate the plastic film and the adhesive layer while only touching or partially penetrating the paper adhesive protector sheet. The paper sheet is discarded during subsequent operation.
A variable drive for the patterned roll permits speed changes in the web as various power-speed combinations are tested.
At maximum energy input from available ultrasonic source, the speed of the laminated plastic strip, up to approximately 220 linear ft/min produces satisfactory results when using a plastic web width of approximately three inches. Since a finite amount of ultrasonic energy is utilized over an area per unit of time; i.e., width.times.length=constant, a narrower web width would be processable at a faster linear speed and a wider web at a slower rate. It is also assumed that the energy required to produce perforations would be proportioned to the thickness of the web to achieve the same rate of production.